1. Field of Invention
This invention relates to an optical information recording medium for recording and reproducing information on being irradiated with laser beams, more particularly a phase-changing optical disk recording information on a land and groove as the recording tracks formed by a guiding groove.
2. Description of the Prior Art
An optical information recording medium, in particular an optical magnetic disk and phase-changing disk, generally has recording tracks formed by concentric or spiral guiding grooves formed on the substrate. Pitch of the tracks has been decreasing to meet demands for increased recording density. One of the techniques therefor is land-groove recording in which information is recorded on both concave (groove) and convex (land) sections for each guiding groove. However, the land-groove recording method involves problems coming from difference in recording sensitivity between the land and groove sections. One of these problems is possible defective recording resulting from insufficient phase change at the side of lower recording sensitivity, when information is recorded on both sections by the same recording power. Furthermore, there occur such a problem that it is necessary to change power for recording on the land and groove, in order to realize optimum recording on each section, which makes the recording device more complex.
The above phenomenon will be explained by referring to FIG. 3, which shows a sectional view of a phase-changing optical disk. The land L and groove G, formed by the guiding groove 37 on the substrate 31, are laminated with the lower dielectric layer 33, recording layer 34, upper dielectric layer 35 and reflection layer 36, in this order. The disk is irradiated with laser beams from the substrate side, which are projected onto the land L or groove G to record information by changing phase of the recording layer 34. As shown by the sectional structure, the reflection layer 36 of high heat radiation adjoins the recording layer 34 via the groove slope on the land L, whereas the lower dielectric layer 33 of low thermal conductivity adjoins the recording layer 34 on the groove G. Therefore, thermal diffusion by heat flow H1 from the recording layer 34 towards the reflection layer 36 is accelerated at both ends of the land L, when it is irradiated with laser beams, by which is meant that a higher recording power is needed to realize phase change at the recording layer 34. This will result in lower recording density on the land L.
One method to solve the difference in recording sensitivity between the land and groove in an optical disk is changed thickness of the reflection, dielectric or recording layer on the land and groove, as disclosed by Japanese Patent Application Laid-Open No. 7-130006. This method changes thickness of the reflection or dielectric layer on the land and groove, to improve recording sensitivity on the land. The invention disclosed by Japanese Patent Application Laid-Open No. 7-311980 tries to enhance recording density, erasing rate and C/N ratio, and also to reduce crosstalk for recording with land-groove tracks by keeping reflectivity of the crystal sections of the land and groove in a range from 7 to 15%, inclusive. The invention disclosed by Japanese Patent Application Laid-Open No. 6-338085 tries to reduce erasing leftovers for magnetic modulation overwriting by keeping a higher recording density on the groove than on the land for a photomagnetic recording medium. The invention disclosed by Japanese Patent Application Laid-Open No. 2-108254 tries to reduce occurrence of erasing leftovers by keeping the metallic layer thicker on the groove than on the land, thereby reducing recording density on the groove to prevent formation of a magnetic domain on the groove and to allow a magnetic domain to be recorded on the land only when a tracking gap occurs.